1. Field of the Invention
The invention relates to extrudable white polyolefin compositions which exhibit reduced die lip build-up during extrusion. More specifically, the compositions contain a low density polyethylene, a whitening or opacifying filler and a die drool-reducing amount of mercaptobenzimidazole compound
2. Brief Description of the Prior Art
Inorganic fillers are commonly incorporated in polyolefin resins for a variety of purposes. For example, inorganic fillers may be employed to reduce cost, impart flame retardancy, improve modulus or toughness, impart opacity, impart printability/writeability, impart ultraviolet (UV) resistance and the like. The amount and type of polyolefin used and inorganic filler incorporated can vary widely depending on the intended end-use application.
Whitening or opacifying inorganic fillers, such as titanium dioxide and calcium carbonate, are widely used in the production of white or opaque polyolefin packaging films. Films of this type may be used individually or incorporated as one of the layers in a coextruded multi-layer film construction. White films are commonly employed where a white background for printing is desired and to protect packaged foods which are sensitive to light exposure.
The problems associated with the high-speed extrusion of filled polyolefins and subsequent forming operations are well documented. For example, U.S. Pat. No. 5,277,970 describes the tendency of opaque, vacuole-containing films to develop undesirable, stripe-like markings of varying intensity in the longitudinal direction when processed on high-speed automatic packaging machines and discloses multi-layered white film resistant to the development of such marking.
The presence of inorganic fillers is also known to contribute to undesirable die lip build-up during extrusion/coextrusion. Dip lip build-up, also referred to as die drool, die drip or plate out, is the undesirable accumulation of polymer on the open face of a die. Die lip build-up can change the geometry of the die exit and cause surface roughness of the extrudates. Also, the built-up material can periodically break free and cause localized surface defects. If die drool is excessive it may require periodic shut down to clean dies and/or it may require operating at reduced line speeds. A discussion of the problems associated with die lip build-up and the various factors which contribute to this phenomenon and a review of common methods suggested to minimize or eliminate die lip build-up problems is provided in a paper presented by J. D. Gander and A. J. Giacomin, SPE ANTEC, Indianapolis (May 1996), pages 1113-1117.
One approach to reducing die drool is the use of additives that function as processing aids/lubricants for the polyolefin. Fluoropolymer additives have been widely used for this purpose. There is a continuing need to provide new and improved white-filled polyolefin compositions which exhibit reduced die lip build-up upon extrusion. It would be particularly desirable if these compositions utilized readily available and low cost additives.
Benzimidazole compounds have been used as stabilizers for polyolefins to prevent degradation caused by oxidative, thermal and ultraviolet exposure. For example, U.S. Pat. No. 3,218,276 discloses polypropylene fiber-forming compositions protected against thermal and ultraviolet degradation containing 0.2 to 2 percent C3-20 alkyl-substituted benzimidazole. U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working. Zinc mercaptobenzimidaole is specifically mentioned.
Combinations of hindered phenols with various zinc salts of mercapto compounds to stabilize crosslinkable polyolefin insulation compositions is taught in U.S. Pat. Nos. 4,260,661; 4,693,9837; 4,797,323 and 4,824,883. For example, combinations of IRGANOX 1010 with the zinc salts of 2-mercaptobenzimidazole and 2-mercaptotoylimidazole are disclosed.
Other references which disclose benzimidazole stabilizers for crosslinkable elastomeric polymeric materials include U.S. Pat. Nos. 4,459,380; 4,808,643 and 5,196,462.
U.S. Pat. No. 6,197,852 discloses the use of mercaptobenzimidazole compounds for carbon black-filled concentrates having improved resistance to thermo-oxidative degradation and improved dispersion of carbon black.